A known method of measuring the flow rate of a fluid (liquid or gas) through a conduit involves measuring the pressure either side of a restriction of constant area in the conduit. If the pressure drop across the restriction is recorded for a variety of known flow rates then the function of the flow rate with respect to the pressure drop can be determined. The flow rate is basically related to the pressure drop by a quadratic function (ie. the pressure drop is proportional to the square of the flow rate). When the function is known then, by measuring the pressure drop, it is possible to calculate the corresponding flow rate. Devices capable of measuring the pressure drop include, for example, electronic differential pressure transducers and fluid manometers.
Due to the quadratic relationship, small variations or errors in the measuring of the pressure drop at low flow rates produce large variations or errors in the calculated flow rate. Restrictions of constant area also produce a large pressure drop at high flow rates. Accordingly, a restriction of constant area is unsuitable when a large range of flow rates is required to be measured, especially where accuracy is required at the lower end of the range.
An example of an application where the above properties are undesirable is the flow rate measurement of air (or other breathable gas) supplied to a patient undergoing continuous positive airway pressure (CPAP) treatment for obstructive sleep apnea. In particular, when the pressure of the gas supplied to the patient is bi-level (in synchronism with patient inspiration and expiration) or autosetting in level, then accurate flow rate readings of down to zero flow are required for triggering purposes by a control system. Also, the flow rate must also be able to be measured at peak flows of up to about 200 liters per minute without a large pressure drop being caused.
A restriction of variable area can ameliorate some of the above problems. A prior art variable area restriction includes a resilient plastic flap that, in an unstressed state, almost occludes a restriction in the conduit. The flap deflects to enlarge the permitted flow area of the conduit under the influence of the fluid flowing through the restriction. The higher the air flow, the more the flap deflects, and the larger the restriction area becomes. The resilient flap can be configured to provide an almost linear relationship between pressure drop and flow rate over a useful range of flows. In this way, the resilient flap provides the desired level of accuracy at both relatively low and high flows. Further, as the area of the restriction is increased at high flows, the resilient flap does not cause a large pressure drop at these high flows.
However, the resilient flap suffers from the disadvantage that, after continuous use, it can take on a permanently deflected set and therefore provide erroneous or inaccurate readings at low flow rates.
It is an object of the present invention to substantially overcome or at least ameliorate these prior art deficiencies,